add ability to break scales

[davelandry]

Some good examples:

• https://community.tableau.com/thread/142805
• https://www.stata.com/support/faqs/graphics/scale-breaks/
• https://www.graphpad.com/support/faq/graph-tip-how-to-put-a-gap-or-gaps-in-an-axis/

I'm thinking of it being implemented through methods like xBreak and yBreak, passing a 2-length Array in order to define the break (from start to finish).

Things to implement:

• allow breaks in axis d3plus-axis#60
• mask over the shapes at the area of the break